The present disclosure relates to gas turbine turbo-shaft engines and more particularly to a system and method for transfer to a backup system in response to an automatic engine control failure.
Gas turbine engines often employ Full Authority Digital Engine Control (FADEC) systems for engine management. Traditionally, these are one or two channel configurations with a backup capability to control the engine manually following total loss of automatic control. These manual backup control systems may be relatively expensive, complicated and impose a weight penalty.
With the advent of more reliable and redundant automatic engine control systems, the requirement for a manual backup system has diminished to the extent that many integrated designs delete the manual backup system entirely. Consequently, complete loss of automatic control results in immediate shut down of the affected engine. Statistical analyses of various failure modes and probabilities have supported removal of the manual backup system for commercial aircraft applications, but such rational may not be applicable to combat aircraft where the probabilities for total loss of engine control include other potentialities such as ballistic vulnerabilities; accommodation of redundant controls in a single unit; exposure to intense electromagnetic radiated fields; severe environmental conditions; and other considerations.
Combat aircraft often retain some form of manual backup systems. Operation thereof, however, requires significant coordination between the pilot and copilot. This may be further complicated during particular tactile flight profiles. In addition, manual operation and control may become complicated on gas turbine engines that utilize a variable inlet guide vane (IGV) system.
Given these requirements, some manual backup systems will “fail-fixed” to the fuel flow/power level present at the moment of automatic control loss. Gas turbine engine fuel control systems generally have a steady-state fuel flow boundary and an over/under transient range necessary to achieve maximum acceleration or deceleration of the engine. The more responsive the engine, the larger the transient boundaries. If the engine should fail-fixed within the high fuel flow transient boundary, the engine may eventually suffer damage. Should the engine fail-fixed within the low fuel flow transient boundary, the engine may flame-out.
Furthermore, if the loss of automatic control occurs within the high fuel flow transient boundary, any reduction in power required may result in main rotor system or engine overspeed. That is, if the automatic control fails within the high fuel flow transient boundary, the fuel metering valve is likely in a period of acceleration. By the time the automatic control defaults to the manual backup system, the residual FADEC high fuel demand and the basic inertia of the metering valve system itself will result in a very high, fuel fail-fixed condition. To compensate for the resultant main rotor system or engine overspeed, the aircrew may have to pull collective to absorb the excessive power. This will thereby increase altitude. The altitude increase may require exit from a nap of the earth flight profile and may potentially expose the aircraft to other threats. In addition, to control rotor overspeed, the engine may be operated outside a permitted operation range with respect to speed and temperature and may thereby suffer damage.
Alternatively, if the loss of automatic control occurs within the low fuel flow transient boundary, the result may be essentially equal to an engine shut down. That is, if the automatic control fails during the low fuel transient boundary, the fuel metering value is in a period of deceleration. By the time the automatic control defaults to the manual backup system, commanded fuel flow may be relatively low; lower than the steady-state fuel flow required to maintain the engine on-line. The aircrew has minimal control to increase engine power on the faulted engine and any power required increase will be placed on the other healthy engine(s).
Some manual backup systems may include manual adjustment of the engine power within a limited range through “beeper” trim commands. This manual adjustment requires direct aircrew interaction in which attention may be directed to engine control rather than the flight regime. Furthermore, the limited range of control through the beeper does not provide engine inlet guide vanes (IGVs) operation such that movement within this limited range of control is relatively slow to minimize the potential for engine flame-out or compressor surge.
Current manual control systems may thereby not provide sufficient engine control during the critical moments which follow automatic control failure.